First post here - I thought I'd give it a try because I've posted this on Apple's forums as well as Digital Photography section on MacRumors.com (has been my "regular" forum for years) and have yet to get any replies at all after hundreds of views

I've read some extensive threads here on Luminous Landscape and was really impressed by the depth and quality of discussions here, so hopefully I can finally get this question answered!

---In Aperture (version 3.2.1, though it's been happening for well over a year and I just never got around to asking), I noticed something perplexing about it's white/black clipping overlays (View > "Highlight Hot & Cold Areas").

I will say right away that my Preferences for "Hot Area threshold" and "Cold Area threshold" are at 100% and 0%, respectively - so that's not the problem.(so at this setting, Aperture should only be highlighting clipping if RGB & Luminance levels are 0 or 255)

Since screenshots taken in OS X don't show the mouse cursor, I've uploaded a sample RAW file if you want to follow along as I'm describing it...http://cl.ly/BoUD

The Aperture puts the white balance initially at 5487k and at this setting, the clipping overlays show "properly" - only pixels of 255 RBG/L values are highlighted as clipping.

However, I notice when I make a white balance adjustment, this doesn't seem to be the case, despite the setting in Preferences.

I'll use a more extreme example to illustrate my point...let's change the white balance color temp to 9000k. At this point, you should see a clipping overlay over the person's nose. However, if you mouse over the overlay on his nose, there are plenty if pixels that have not reached a 255 value that are highlighted as "clipping" (ie. R249 G224 B180 L226).

I think your perception of hot and cold clipping as translating to absolute black and white is what is confusing you. The Aperture manual describes this type of clipping as color information that is "beyond the standard limits of the Aperture working color space." According to this standard, the areas marked by Aperture's hot and cold mask don't necessarily mean they are lacking in pixels, but that those pixels may not produce accurate results within the working color space.

Would you happen to know if Aperture's Highlight Hot & Cold Areas feature adapts based on the soft proofing color space that's selected when it's toggled on & off? I think it "should" (as that is one of the main reasons why one would soft proof - to see if there are any colors clipping).

So based on the assumption that it does adapt to the selected proofing color space - if what you are saying is the case (pixels are being marked as clipped "because it's beyond the standard limits of the Aperture working color space"), since Aperture's working color space is pretty massive (I'm told it's comparable in size to Profoto but not exactly the same), if it's already clipping colors in Aperture's native color space, shouldn't there be massive clipping going on when I proof with sRGB in Aperture? Instead, the Highlight Hot & Cold Areas overlays don't change at all when I toggle proofing in sRGB on and off